[AD] Support for non-blocking menus

[Eric Botcazou <ebotcazou@xxxxxxxxxx>]

The following patch
	http://www.multimania.com/ebotcazou/dev/update_menu.zip
implements the support for non-blocking menus we discussed some time ago.

It splits the former do_menu() into init_menu(), update_menu() and 
shutdown_menu() and reimplements it by means of these three new functions. It 
basically transforms recursive allocations on the stack by do_menu() into 
successive allocations on the heap of a MENU_PLAYER structure that holds the 
state of each nested menu and is updated on each call to update_menu().

Following Bob's suggestion, it also redirects update_dialog() to update_menu() 
whenever a d_menu_proc dialog is activated, thus turning update_dialog() into 
a true non-blocking function too.

Tested under DOS, Windows, Linux and commited. The code is probably not 
reentrant, but its current shape is IMHO good enough for a first try.

-- 
Eric Botcazou